Reversibility of Ferri-/Ferrocyanide Redox During Operando Soft X-Ray Spectroscopy

Reversibility of Ferri-/Ferrocyanide Redox during Operando Soft X-ray Spectroscopy The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Risch, Marcel, Kelsey A. Stoerzinger, Tom Z. Regier, Derek Peak, Sayed Youssef Sayed, and Yang Shao-Horn. “Reversibility of Ferri-/ Ferrocyanide Redox During Operando Soft X-Ray Spectroscopy.” The Journal of Physical Chemistry C 119, no. 33 (August 20, 2015): 18903–18910. As Published http://dx.doi.org/10.1021/acs.jpcc.5b04609 Publisher American Chemical Society (ACS) Version Author's final manuscript Citable link http://hdl.handle.net/1721.1/109590 Terms of Use Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. Reversibility of Ferri-Ferrocyanide Redox During Operando Soft X-Ray Spectroscopy ┴ Marcel Risch,†* Kelsey A. Stoerzinger,§ Tom Z. Regier,‡ Derek Peak,º Sayed Youssef Sayed,† Yang Shao-Horn†§||* †Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA, USA 02139 §Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA 02139 ||Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA 02139 ‡Canadian Light Source, Inc., Saskatoon, SK, Canada S7N 2V3 ºDepartment of Soil Sciences, University of Saskatchewan, Saskatoon, SK, Canada S7N 5A8 KEYWORDS. Electrochemistry; In-situ; redox shuttle; iron cyanide; X-ray absorption; radiolysis; radiation damage. ABSTRACT The ferri-ferrocyanide redox couple is ubiquitous in many fields of physical chemistry. We studied its photochemical response to intense synchrotron radiation by in-situ X-Ray absorption spectroscopy. For photon flux densities equal to and above 2×1011 s-1mm-2, precipitation of ferric (hydr)oxide from both ferricyanide and ferrocyanide solutions was clearly detectable despite flowing fast enough to replace the solution in the flow cell every 0.4 s (flow rate 1.5 ml/min). During cyclic voltammetry, precipitation of ferric (hydr)oxide was promoted at reducing voltages and observed below 1011 s-1mm-2. This was accompanied by inhibition of the ferri- ferrocyanide redox, which we probed by time-resolved operando X-ray absorption spectroscopy. Our study highlights the importance of considering both electrochemical and spectroscopic conditions when designing in-situ experiments. Introduction Ferri-ferrocyanide is a common facile redox couple1-3 that is frequently used to access surface charge transfer kinetics.4-7 It is also an important component of diverse applications, including in charge storage within flow batteries,8 as the electroactive species in thermogalvanic cells,9-11 a preventor of electrolyte decomposition in batteries,12 and as the redox shuttle in dye-sensitized solar cells13 and photoanodes.14 Previously, the redox of ferricyanide and ferrocyanide has been studied by hard and soft in-situ X-ray absorption spectroscopy (XAS) to elucidate photo- 15-16 17 18-19 20 excitation, chemical reduction, solvation, and charge transfer. The attenuation length of hard X-rays at the Fe K-edge (7112 eV) is ~100× larger than that of soft X-rays at the Fe L3-edge (706.8 eV),21 which enlarges the irradiated mass by a factor of ~ 100 for hard X-rays, while the energies differ by a factor of ~ 10. The dose (energy deposited per mass) within one absorption length is then about 10× larger at the Fe L3-edge as compared to the Fe K-edge for a fixed number of incident photons. (Exact calculation gives 6.6× larger dose.22) Therefore, less radiation damage can be expected at the Fe K-edge as compared to the Fe L3-edge. Yet there is little discussion of photochemical effects on the stability of ferricyanide and ferrocyanide during operando XAS with soft X-ray radiation. Moreover, Ferricyanide and ferrocyanide complexes can become unstable due to CN cleavage under UV illumination,23 and electron irradiation,24-26 especially at high pH.27 These interactions lead to drastically reduced kinetics after few hours of operation,28 which is commonly explained by precipitation of coordination polymers related to 28-31 32 Prussian Blue (ferric hexacyanoferrate) with general formula AhFek[FeCN]l·mH2O, where A is a group I metal and h, k, l, m are stoichiometric indices. Here, we studied the implications of ionizing radiation on ferricyanide and ferrocyanide complexes during open-circuit and cyclic voltammetry using soft XAS. Our experimental setup allows studying reactions within the bulk electrolyte and any side reactions occurring at the surface of the working electrode. This is in contrast to previous studies of iron sulfate redox in the bulk electrolyte,33 where surface reactions at the working electrode are not probed by XAS (Figure S1). We find the threshold photon flux that can trigger the radiolysis of ferricyanide to form ferric (hydr)oxide at the electrode surface under open-circuit. Conducting cyclic voltammetry below the threshold photon flux resulted in the precipitation of ferric (hydr)oxide and electrode passivation in the irradiated area. Our work highlights the importance of considering both photochemical and electrochemical damage when designing electrochemical operando XAS experiments. Experimental X-ray absorption spectroscopy X-ray absorption measurements at the iron L-edges were performed at the spherical grating monochromator (SGM) beamline 11ID-1 at the Canadian Light Source.34 The window of the sample cells was mounted at an angle of roughly 45º with respect to both the incident beam and the detectors.35 The irradiated area on the cell window was about 0.05 mm2 as determined by image analysis in Adobe Photoshop (Figure S2). All measurements were made at room temperature in the fluorescence mode using Amptek silicon drift detectors (SDD) with 1024 emission channels (energy resolution ~120 eV). Four SDD were employed simultaneously; two detectors had vanadium (200 nm) and two detectors had titanium (200 nm) filter foils mounted to suppress the oxygen fluorescence. The partial fluorescence yield (PFY) was extracted from all SDDs by summation of the iron L emission lines between 664 and 872 eV. The PFY spectra were normalized for the background due to oxygen absorption by fitting a straight line in an appropriate region below the L3-edge (typically between 695 and 703 eV) and subtracting it over the whole range of the data (685-755 eV) as shown in Figure S3A. The noise level of some spectra required adjustment of the boundaries so that the post-edge slope matched the pre-edge slope reasonably well. Finally, the average intensity between 732 and 735.5 eV (after L2 edge) was normalized to unity (Figure S3B). The energy axis was calibrated with respect to the pre- edge in the spectrum of molecular oxygen at 530.8 eV,36 which was acquired using a sample cell filled with ambient air. The incident intensity was obtained by measuring the current on a gold grid placed before the sample chamber. The absolute incident flux of 1×1010 photons/s was measured using a photodiode (IRD AXUV100) for a beamline exit slit size of 10 µm.37 We assumed a linear dependence of the current on the gold grid and the photon flux within the used range of 50% to 150% flux. Flux densities were obtained by division of the irradiated area on the cell window (0.05 mm2). In the ionic limit relevant to L-edge spectroscopy of iron, the spectral features are dominated by multiplet interactions between the ground, excited and relaxed electronic states,38 which can provide a fingerprint for ferri- and ferrocyanide that can be assigned uniquely to an electronic 5 0 III 3- configuration, e.g., t2g eg . The iron L-edge spectra of ferricyanide, [Fe (CN)6] , and II 4- 19, 39 ferrocyanide, [Fe (CN)6] , have been analyzed in detail by theoretical methods and complementary experiments using high-resolution resonant inelastic X-ray scattering (RIXS).18, 40 Sample cell The bodies of the sample cells were fabricated on an Object Connex500 printer by 3D printing with DurusWhite material (Figure 1). The design of the electrochemical flow cell was adapted from a previously used flow cell.41 Silicon nitride membrane windows (1 mm x 1 mm x 100 nm) in Si frames (5 mm x 5 mm x 525 µm) were purchased from Silson or SPI Supplies. For flow cells, the windows were used as received. For electrochemical flow cells, the windows were treated by HF, and then coated by electron-beam evaporated carbon (10 nm) and gold (15 nm). The gold-covered window was contacted by gold wires. Platinum wires were used as reference and counter electrodes. In the final step of assembly, the windows were glued to the cell body using Varian Torr seal epoxy, forming an electrolyte chamber of ~ 1 mm height. The contact between the gold film on the windows of the electrochemical cell and the gold wire was monitored during the curing process of the epoxy. Only cells with resistance between the two 21 gold wires below 120 Ohm were used for electrochemical experiments. Using the Henke tables, -1 we calculated attenuation lengths (I/I0 = e ) at 715 eV and 45º of 560, 40, 360 and 720 nm, for C, Au, Si3N4 and water (i.e. ice), respectively. The resulting transmission through these components can be found in Figure S4A. About 82% of the intensity outside the sample cell was available at the window of the flow cell (schematic in Figure 1A) and 63% is available at the Au electrode surface of the electrochemical cell (schematic in Figure 1B). While incident X-rays penetrate deep into the bulk of the electrolyte, most of the detected fluorescence likely escaped from regions closer to the electrode due to reabsorption in the electrolyte and subsequent isotropic emission, of which a dwindeling fraction can be expected to arrive at the detector with increasing distance to the electrode.

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